Optical fibers have become a desirable information transmitting medium due to their broad bandwidth capacity and small physical size and weight relative to metal electrical conductors. A number of characteristics of optical fibers including their susceptibility to breakage and their bending and stress losses pose serious problems in their use. It is therefore, necessary to find suitable means to protect the fibers.
One approach to the above problems has been to start with a plurality of optical fibers and to form them into linear arrays packaged in ribbon-like structures. This approach is described in detail in U.S. Pat. No. 4,129,468, issued Dec. 12, 1978. The requirements imposed on such optical fiber ribbons include the need to provide mechanical stripability for ease of cable termination and splicing; the need for small size; the need for resistance to breakage when subjected to tensile stress; the need for individual fiber identification within the ribbon; and, the need to protect the ribbon from distorting forces which may cause deterioration of the optical signal.
After forming these ribbons, they are used to form optical cables. It has been asserted that these ribbons provide adequate protection for the optical fibers when used for information transmission. Various processes for manufacturing such an optical cable are described in U.S. Pat. Nos. 3,937,559; 4,110,001; 4,138,193 and 4,146,302. As appears in each of these patents, it is still necessary to form the ribbon-like structure prior to the cabling operation. Indeed, it is specifically stated in each of these patents that forming of the ribbon-like structure reduces the risk of injury to the optical fibers during the cabling operation. The drawback to such a requirement is particularly significant when a cable having only a few fibers is required. It is an unduly burdensome procedure to first produce ribbons and then make a cable to carry the ribbon. The cabling of the optical fiber ribbons is a complicated and expensive procedure, even after the ribbons have been assembled. Cabling of these ribbons requires the use of planetary stranders at least for the laying of strength members into the required extruded sheath. Such equipment is expensive; and hence, contributes significantly to the overall cost of the cabling operation.
It has been proposed to avoid the requirement for a multistep cabling process by incorporating one or more optical fibers into a cable as the cable was formed. Such procedures are described in U.S. Pat. Nos. 4,155,963; 4,154,049; and 4,205,899. These patents disclose the extrusion of a profiled central member having grooves into which optical fibers are laid, followed by closure of the groove, thereby enclosing the fiber in a longitudinally extending chamber. U.S. Pat. No. 4,199,224 is similar and adds a separate central strength member. These methods permit optical fibers to be laid into the open channels as the cable is manufactured; hence, eliminating the necessity of first manufacturing an optical fiber ribbon. The methods disclosed in these patents require expensive machinery and a large amount of floor space for cabling of optical fiber. As a result of the expenses associated with the machines and dedicated floor space, these are expensive cabling techniques.
Another approach for cabling of optical fibers is disclosed in U.S. Pat. No. 4,153,332 issued May 8, 1979. This patent explains that a cable may be formed by winding unitary elements having an adhering sheath on a supporting core. When this structure is bent on a mandrel having a small bending radius, the unitary element is compressed in the inner portion facing the mandrel and is stretched in the outer portion. This occurs because of the fact that the friction between the element and the core about which it is wound prevents the element from sliding significantly with respect to the core. The optical fiber or fibers contained in the unitary element are subjected to compressive and tension stresses. To overcome this drawback, it is recommended that the fibers be centered in a tubular sheath. This patent discloses that, at the present state of the art, no processes are known for producing unitary optical fiber elements which permit a perfect centering of the optical fiber with respect to the sheath. This is especially true when the sheath has a diameter much greater than that of the fiber. When the fiber is not centered, it is well known to those skilled in the art that the tension or compressive stress on the fiber, when the unitary element is subjected to flexing, is proportional to the distance of the optical fiber from the neutral axis of the unitary element and inversely proportional to the bending radius of the element. To compensate for this, the fibers may be longer than the length of the corresponding surrounding sheath by either winding the fibers around a central core within the sheath or by imparting a helical bend to the fibers and laying them loosely within the sheath. The patent goes on to assert that the provision of a tubular sheath into which the optical fiber is inserted provides a protective structure which shields the surface of the fiber from radial compressive force and from contact with corrosive substances.
Another approach which has been proposed for providing a protective cable structure for optical fibers is disclosed in U.S. Pat. No. 4,235,511 issued Nov. 25, 1980. This patent describes a cabling technique wherein a central strength member is surrounded by elements which define chambers running the length of the strength member and which are covered in order to enclose an optical fiber laid into the chamber. The patent discloses folded splicing tape helically wrapped about the central strength member for formation of the chambers.
All of the above described techniques suffer from one or more of the problems including high expense in cable manufacture, susceptibility to fiber breakage during cabling or cable laying operations, and poor optical transmission characteristics.